CONCENTRATIONS OF THYROID-HORMONES AND IODOTHYRONINE BINDING-PROTEINSIN SERUM OF THE KOALA (PHASCOLARCTOS-CINEREUS)

Objective To examine circulating total and free thyroid hormone (T-3 a nd T-4) concentrations, determine serum iodothyronine binding characte ristics and estimate thyroid stimulating hormone (TSH) activity in ser a of coastal and inland koalas. Design A prospective study. Procedure Koala serum T-3 and T-4 were measured by radioimmunoassay. T-4 binding parameters were determined by radioligand binding and electrophoresis . Koala TSH values were determined by bioassay. Results Mean total T-4 concentrations were 3.2 +/- 2.1 nM although values were significantly higher in inland-dwelling females in comparison to coastal-dwelling m ales. Free T-4 was 3.3 +/- 2.1 pM. Total and free Tg were 0.4 +/- 0.2 nM and 1.4 +/- 0.9 pM respectively, although these values were at the lower end of the assay detection limit and should be viewed with reser vation. Electrophoresis of [I-125]-T-4-labelled serum revealed only tw o proteins of electrophoretic mobility similar to human transthyretin (TTR) and albumin. Scatchard analysis of T-4 binding to serum gave a c urvilinear plot, which could be resolved into two binding sites with a ffinities identical to that of TTR and albumin but both of low concent ration. The bioactivity of the TSH present in the sera was measured us ing a cell line (JP09) transfected with the human TSH receptor. The me an level of stimulation found in the sera corresponded to a bovine TSH activity of less than 10 mU/L. Conclusion These results suggest that the serum concentrations of free and total thyroid hormones in koalas are low compared to other marsupials and very low compared to eutheria n mammals. The mechanism of maintenance of euthyroidism in this specie s remains to be determined.